The present invention relates to a solid microlaser, a cavity for said microlaser and a process for the production of said cavity.
One of the advantages of the microlaser is its structure in the form of a stack of multilayers. The active laser medium is constituted by a material of limited thickness between 150 and 1000 xcexcm and small dimensions (a few mm2), on which are directly deposited dielectric cavity mirrors. This active medium can be pumped by a III-V laser diode, which is either directly hybridized on the microlaser, or coupled to the latter by an optical fibre. The possibility of collective production using microelectronic means allows a mass production of such microlasers at very low cost.
Microlasers have numerous applications in fields as varied as the car industry, the environment, scientific instrumentation and telemetry.
Known microlasers generally have a continuous emission of a few dozen mW power. However, most of the aforementioned applications require peak powers (instantaneous power) of a few kW delivered for 10xe2x88x928 to 10xe2x88x929 seconds, with a mean power of a few dozen mW. In solid lasers, it is possible to obtain such high peak powers by making them operate in the pulsed mode at frequencies between 10 and 104 Hz. For this purpose use is made of well known switching methods, e.g. the Q-switch.
More specifically, the switching of a laser cavity consists of adding to it time-variable losses, which will prevent the laser effect for a certain time during which the pumping energy is stored in the excited level of the gain material. These losses are suddenly reduced at precise moments, thus feeing the stored energy in a very short time (giant pulse). Thus, a high peak power is obtained.
In the case of so-called active switching, the value of the losses is externally controlled by the user (e.g. intracavity electrooptical or acoustooptical, rotary cavity mirror changing either the path of the beam, or its polarization state). The storage time, the time of opening the cavity and the repetition rate can be independently chosen. However, this requires adapted electronics and makes the laser system more complicated. An actively switched microlaser is described in EP-724 316.
In the case of so-called passive switching, variable losses are introduced into the cavity in the form of a material known as a saturable absorber (S.A.), which is highly absorbant at the laser wavelength and has a low power density and which becomes virtually transparent when said density exceeds a certain threshold, which is called the S.A. saturation intensity.
A passively switched microlaser is described in EP-653 824.
The latter document more particularly describes a microlaser having:
a solid active medium, which can be constituted by a base material chosen from among Y3Al5O12, LaMgAl11O19, YVO4, Y2SiO5, YLiF4 or GdVO4, doped with erbium (Er) or an erbium-ytterbium (Erxe2x80x94Yb) codoping),
a saturable absorber deposited by liquid phase epitaxy directly on the solid active medium and constituted by a base material, identical to that of the solid active medium, and doped with Er3+ ions.
This microlaser makes it possible to obtain an emission length of approximately 1.5 xcexcm. This emission length has a particular interest, particularly in the field of optical telecommunications. The diversity of specific applications in this field makes it necessary to have other microlaser sources making it possible to emit at or close to this wavelength.
The article by M. B. Camargo et al entitled xe2x80x9cCo2+:YSGG saturable absorber Q-switch for infrared erbium lasersxe2x80x9d, published in Optics Letters, vol. 20, No. 3, pp 339-341, 1995, describes; a laser passively switched with the aid of a saturable absorber Co2+:Y3Sc2Ga3O12 and a saturable absorber Co2+:Y3Al5O12. Thus, it is a saturable absorber based on YAG or YSGG and doped with Co2+. However, for the reentry of a 2+ charging ion, it is necessary to carry out a charge compensation of the substrate with a 4+ ion (charged four times positively), in order to maintain the neutrality of the compound. This charge compensation problem is made all the more difficult to solve in that the crystals described in the article by M. B. Carmago et al are firstly produced in solid form and are then cut up into sections. Moreover, the method described in this document for producing the crystal (Czochralski method) limits the concentration of dopants which it is possible to introduce into the matrix, as a result of stability problems. Finally, this document provides no specific construction for an operation with a high repetition frequency.
In order to solve these problems, the invention relates to a monolithic, solid microlaser emitting at at least one wavelength in the infrared range exceeding 1.5 xcexcm and which supplies a pulsed beam by a passive switching process. Such a device will preferably operate at a high repetition frequency. The microlaser is formed from at least two materials, namely a solid amplifier medium and a saturable absorber medium, or which serves the purpose of a saturable absorber, i.e. a self-modulated loss modulator.
The invention relates to a microlaser cavity having a solid active medium emitting at least in a wavelength range between 1.5 and 1.6 xcexcm and a saturable absorber of formula CaF2:Co2+ or MgF2:Co2+ or SrF2:Co2+ or BaF2:Co2+ or La0.9Mg0.5-xCoxAl11.433O19 or YAlO3:Co2+ (or YAl5-2xCoxSixO3 YAl(1-2x)CoxSixO3) or Y3Al5-x-yGaxScyO12:Co2+ (or Y3Al5-x-y-2zGaxScyCOzSizO12) or Y3-xLuxAl5O12:Co2+ (or Y3-xLuxAl5-2yCoySiyO3) or Sr1-xMgxLayAl12-yO12:Co2+ (or Sr1-xMgx-yCoyLazAl12-zO12, with o less than y 21 x) Sr1-xLaxMgxAl12-xO19:Co2+ (or Sr1-xLaxMgx-yCoyAl12-xO19, with 0 less than y less than x).
All these saturable absorber compositions make it possible to obtain a saturable absorber element in thin film form, e.g. by molecular beam epitaxy, or by sol-gel deposition. The films have the advantage of being stressed more easily than solid structures.
Moreover, the compositions CaF2:Co2+, MgF2:Co2+, SrF2:Co2+, BaF2:Co2+ or La0.9Mg0.5-xCoxAl11.433O19 do not require a charge compensation, due to the introduction of cobalt as a dopant. Consequently, the saturable absorber can be a film with a thickness between e.g. 1 and 200 xcexcm (e.g. between 5 and 150 xcexcm).
A microlens can be provided on one of the faces of the microlaser, which makes it possible to stabilize the cavity and helps to lower the operating threshold of the microlaser.
The saturable absorber can be combined with the amplifier medium or solid active medium by various processes:
by assembly with an optical adhesive or a resin bead,
by deposition in film form (e.g. a Co2+-doped sol-gel film, or a Co2+-doped, epitaxied film),
or by a mixture of the two processes involving firstly a deposition on a substrate which is not the laser material, followed by the assembly of said substrate with the amplifier medium, after which the film substrate can be removed, or it can already contain one of the mirrors of the microlaser cavity.
Thus, the saturable absorber can be a material of different types:
a crystal doped with a saturable absorber ion (e.g. Co2+-doped LMA),
a monocrystalline film doped with a saturable absorber ion (e.g. LMA:Co2+, or CaF2:Co2+ or MgF2:Co2+ or SrF2:Co2+ or BaF2:Co2+), e.g. deposited by liquid phase epitaxy on the laser crystal, which is matrix or stress-matched,
a sol-gel film deposited on the laser material and doped with a Co2+ saturable absorber ion.
The microlaser or microlaser cavity according to the invention can operate at a high repetition frequency (xe2x89xa7100 or 200 Hz).
The invention also relates to a process for the production of a device as described hereinbefore.